This application claims the benefit of the filing date as provided by 35 U.S.C. 119 of European patent application number EP-02001096.3 filed on Jan. 22, 2002, the disclosure of which is incorporated herein by reference.
The present invention relates in general to laser drivers coupled to laser modules for modulating the laser output thereof. In particular, the present invention relates to solutions enhancing the operation of laser drivers being AC coupled to laser modules.
The optical output power of laser diodes varies as a function of temperature and operating time. While temperature effects on the optical output power can be compensated for by cooling of laser diodes, for example by means of cooled laser modules including laser diodes, aging effects require some kind of regulation loop. To compensate the impact of aging it is known to employ a laser module including a monitor diode for monitoring the optical output power of a laser diode. Suitable monitor diodes are optical sensors providing a photo current being representative of the present optical output power of the laser diode. On the basis of the output of the monitor diode, the present optical laser output power is determined to adjust a current used as power supply for the laser module and the laser diode, respectively.
Such a measure for compensating aging effects is also employed for laser modules wherein laser diodes included therein and, in particular, the optical output power of laser diodes is modulated. Depending on the type of laser module, two main principles for modulating the optical laser output power are used in practice. According to one principle, a modulation voltage is applied to a modulator integrated in a laser module while a laser diode thereof itself operates in a continuous wave (CW) mode at a constant so-called laser current, i.e. a current used as power supply for the laser diode and determining the optical laser output power thereof. According to the other principle, a laser diode itself is modulated by means of a modulation current. In the latter case a resulting current actually determining the laser output power can be considered as sum of a laser current usually being a fixed value and a modulation current.
Available laser drivers for a direct modulation of a laser diode by a modulation current are designed for a so-called DC coupling between the laser driver and the laser diode. FIG. 1 illustrates such a DC coupling of a laser driver and a laser module. The laser module includes a laser diode LD, a unit for temperature compensation, such as a thermoelectric cooler and a thermistor for stabilizing the temperature of the laser diode, and a monitoring unit including a photo diode PD which is optically coupled to the laser diode LD.
As set forth above, temperature compensation can be properly achieved by means of the temperature compensating unit, while compensation of aging effects can be obtained by a determination of the optical laser output power. For that purpose, by means of the photo diode PD in response to light from the laser diode LD being dependent from the optical laser output, power the monitoring unit provides a measure, i.e. a photo current, which characterizes the actual optical laser output power.
For a DC coupled operation of the laser driver shown in FIG. 1, the current actually determining the optical output power is a sum of a supply current Isource for the laser module and a part of a modulation current Imod supplied from the laser driver. This relation is illustrated in FIG. 2.
Thus, a nominal laser current providing a nominal optical output power Pnom is a sum of the supply current Isource and a part of the modulation current Imod, as shown in FIG. 2.
In response to aging effects of the laser diode, the curve characterizing the optical output power P will drive horizontally towards higher current values, while its slope will generally remain unchanged. This phenomena is illustrated in FIG. 2 by the solid curve representing the initial optical output power characteristics and the dashed curve representing a varied optical output power characteristics due to aging impacts.
Thus, it is complicated to obtain stable operating conditions for such a DC coupled arrangement of a laser driver and a laser module since both the supply current Isource and the modulation current Imod have to be modified in order to obtain an age adjusted nominal optical output power Pnom. Moreover, it is necessary to modify the supply current Isource and the modulation current Imod in such a manner that the extinction ratio for the respective laser module is maintained, i.e. kept constant, which is an absolute requirement in most laser optical transmission systems. Due to this aspects, it is necessary to know the actual aging factor.
For DC coupled arrangement of laser drivers and laser modules, the only possible solution is a so-called pilot tone technique for keeping the modulation of the laser diode constant. In the pilot tone technique, a very low frequency signal is added to the modulation current Imod. As a result, the laser current for driving the laser diode comprises the supply current Isource and a part of the modulation Imod, the latter of which being labeled by the low frequency signal. Thus, a photo current provided by the monitoring unit also includes a current portion comprising the low frequency signal or a signal being indicative thereof. By means of filtering measured performed with respect to the current portion including the low frequency signal, it is possible to determine the contributions of the supply current Isource and the part of the modulation current Imod for the laser current driving the laser diode. On the basis of such information, it is possible to maintain the nominal extinction ration and optical output power. However, this approach requires a lot of additional circuitry.
In general, the object of the present invention is to overcome the above problems associated with known arrangements of laser drivers and laser modules. In particular, the object of the present invention is to provide solutions for an improved direct modulation of laser diodes by laser drivers with respect to operation time effects such that nominal optical output power and extinction ratio of a laser diode can be easily maintained.
The basic idea underlying the present invention is to employ an AC coupled arrangement of a laser driver and a laser module as shown in FIG. 3. Here, the laser module and the laser driver, both being comparable to the respective components shown in FIG. 1, are AC coupled by means of a capacitor C. While a supply current Isource for the laser module is supplied comparable to FIG. 1, a modulation current Imod is provided by the laser driver to the laser module via the capacitor C.
As shown in FIG. 4, as a result of the AC coupled arrangement of FIG. 3, the laser current for driving the laser diode necessary to obtain a nominal optical output power Pnom only is a function of the supply current Isource, but not depending from the modulation current Imod. Thus, changes of the optical output power P, for example due to aging effects, can be easily compensated by a respective adjustment of the supply current Isource in order to maintain the nominal optical output power Pnom. For such a compensation, the modulation current Imod is not required to be modified. As a result, a nominal extinction ratio of the laser module can be easily maintained by utilizing the respective modulation current Imod, i.e. the same modulation current Imod independently of a value of the supply current Isource.
In principal, such an AC coupled arrangement of a laser driver and a laser module will solve the object underlying the present invention.
In practice, any laser driver requires a minimum bias current flowing into its output stage, in particular via the output stage port providing the modulation current Imod. This is due to the fact, that all available laser drives for a direct modulation of a laser module and its laser diode, respectively, are designed for the above described DC coupling. In contrast, to the arrangement shown in FIG. 1, no DC current path is existing for the output stage port of the laser driver. Thus, no bias current will be applied to the output stage of the laser driver.
A DC current path to the output stage of the laser driver could be obtained by introducing a bias resistor Rbias as shown in FIG. 5. Here, the bias resistor Rbias is connected between ground and the output stage port. Such an approach would allow for a proper operation of the laser driver with respect to the required bias current. Since the bias resistor Rbias being arranged as shown in FIG. 5 increases the AC load of the laser driver, the maximum available modulation amplitude for the laser module, i.e. the maximum available amplitude of the modulation current Imod, would be decreased. As a result, the current provided via the output stage port of the laser driver would be partitioned between the bias resistor Rbias connected to ground and the modulation current Imod actually supplied to the laser module.
A solution according to the present invention to supply, as an active measure, a bias current to the output stage of the laser driver required for its proper operation. In particular, this is achieved by a bias resistor being coupled to a bias voltage. Moreover, according to the present invention, the bias current being actively supplied to the output stage of the laser driver can be controlled.
As an alternative, the present invention teaches to control the operation of the laser driver in dependence of the modulation current Imod actually supplied to the laser module such that the passive solution, i.e. by means of a grounded bias resistor connected to the laser driver output stage, can be used while the maximum available modulation amplitude of the laser module can still be maintained.
One embodiment according to the invention comprises a laser driver having an output stage which provides a current to be used for a modulation of a laser module and its laser diode, respectively.
Further, this embodiment comprises a unit, for example a capacitor, being connected to the output stage of the laser driver for providing an AC coupling with a laser module. By means of the AC coupling unit, current from the output stage port can be, at least partially, AC coupled to the laser module.
To provide for a bias current to be supplied to the output stage port, as set forth above, a bias current providing unit is coupled to the output stage port.
In particular, the bias current providing unit comprises a bias resistor being coupled to the output stage port, wherein the bias resistor is connected to a bias voltage, which can be a fixed bias voltage.
By coupling the bias resistor to a bias voltage generating unit providing for a variable bias voltage, an enhanced embodiment can be obtained. In particular, the bias voltage generation unit is controlled, directly or indirectly, by a modulation voltage used for the control of the operation of the laser driver and the modulation current, respectively, to be supplied to the laser module.
As an alternative embodiment, the present invention provides an apparatus for modulating a laser module.
This embodiment according to the invention comprises a laser driver having an output stage which provides a current to be used for a modulation of a laser module and its laser diode, respectively.
Further, this embodiment comprises a unit, for example a capacitor, being connected to the output stage of the laser driver for providing an AC coupling with the laser module. By means of a AC coupling unit, current from the output stage port can be, at least partially, AC coupled to the laser module.
To provide for a bias current to be supplied to the output stage port, as set for above, a bias current providing unit is coupled to the output stage port.
In particular, the bias current providing unit comprises a bias resistor being coupled to the output stage port, wherein the bias resistor is connected to ground.
For compensation of the above described decrease of the modulation current actually supplied to the laser module, it is contemplated to use a modulation current sensing unit for determining the modulation current and a modulation voltage generation unit which is controlled in dependence of the determined modulation current. Preferably, the modulation voltage generation unit is controlled by means of an output of the modulation current sensing unit such that the operation of the laser driver compensates a decrease of the modulation current for the laser module resulting from a bias current across the bias resistor. For example, such an operation can be obtained when the laser driver is supplied with a respectively increased modulation voltage.
As an alternate or an option, such an compensation can be obtained by varying the value of the bias resistor being connected to ground. Here it is contemplated to use a modulation current sensing unit, which can be the same as used for the modulation voltage embodiment, to determine the modulation current. In dependence of the determined modulation current, a bias resistor variation unit is controlled such that the bias current necessary for the operation of the laser driver is provided while the required modulation current can be still supplied to the laser module.
For a higher stability and reliability of the operation of the laser driver, the bias resistor can comprise first and second bias resistors, one thereof being coupled to the output stage port, while the other resistor is coupled to the bias voltage or to ground.
Preferably, the first and second bias resistors are connected in series wherein a protection diode is coupled between the first and second bias resistors and connected to ground.
Moreover, the present invention provides a laser module arrangement at least including an embodiment of the apparatus according to the invention. Further, the present invention contemplates methods for modulating a laser module and a computer program product for carrying out at least some of the steps of the methods according to the invention.